1. Field of the Invention
The present invention relates to a cyanoethyl group-containing graft polymer. In particular, the present invention relates to a graft polymer comprising hydrocarbon polymer backbones to which cyanoethyl group-containing side chains are grafted, which has a high dielectric constant and a high ionic conductivity. Such a graft polymer is useful as a binder resin for positive and negative electrodes of lithium ion secondary batteries or for organic dispersion type electroluminescent (EL) devices.
2. Prior Art
Binder resins for electrodes of lithium ion secondary batteries should have good ionic conductivity and also good resistance to a polar solvent (e.g. ethylene carbonate, propylene carbonate, etc.) which is a main component of an electrolyte (hereinafter referred to as "polar solvent resistance" and high adhesion to metal surfaces of electrode collectors.
However, it is difficult for binder resins to have all of such required properties. Thus, fluororesins such as polyvinylidene fluoride resins are unavoidably used. Such resins have low ionic conductivity and poor adhesion to the metal surfaces of collectors, although they have good polar solvent resistance. The ionic conductivity has a large influence on the internal resistance of batteries. Therefore, it is desired to decrease the internal resistance as much as possible to improve the large current discharge properties of the lithium ion secondary batteries and to decrease discharge loss. However, the lithium ion batteries have a larger internal resistance than other secondary batteries such as nickel-cadmium secondary batteries or nickel-hydrogen secondary batteries. This is one of the disadvantages of the lithium ion secondary batteries.
It is desirable for binder resins to have a molecular structure with high polarity, and a high dielectric constant to impart good ionic conductivity to the binder resins for electrodes, but polymers having high polarity have in general low polar solvent resistance. For example, conventional polymers comprising polyoxyethylene backbones or having cyanoethyl groups, which are known to have good ionic conductivity, have very low polar solvent resistance, and thus cannot be used practically. Accordingly, the ionic conductivity and polar solvent resistance are directly opposed properties, and are less compatible.